bug#41531: 27.0.91; Better handle asynchronous eldoc backends

[Dmitry Gutov]

On 03.06.2020 21:07, João Távora wrote:
> The equivalent in futures is just to say clients can set the value nil,
> or some other application-specific indication of "sorry, I failed".

They should call 'eldoc-future-set-error'.

One design point I'm not sure of, is whether the argument should be a 
string (coming from the error message), or a "proper" exception/error 
object, previous captured inside a condition-case.

We might make that choice just based on whether url-retrieve passes the 
same kind of data as, say, error-message-string expects. The same 
"proper" object, that is.